I need to check my answers on part a, and see what's wrong on b. Also, what units do i use?

An abnormal projectile is shot in the air, so that after t seconds, its height in feet is

a) find the velocity at times 0 and 5

b) When is it highest? When does it land?

my answers:

I have the derivative as

a) @t=0, 70.33 (units?)

@t=5, -11.6 (units?)

b) I wasn't totally sure how to do it, so I set the derivative equal to 0. I thought that that should give me the max height, but I came out with -3.34, which is obviously wrong. Did i make a math error or should I not have set it equal to 0? please help!